The world of literature -- verse and rhyme, fable and fairy tales, myths and legends -- is incredibly rich and diverse. The sum total of humanity's outpourings of expression into printed and published texts is vast; within that huge repository of words, sentences, ideas, phrases and sayings are an interconnected web of references to each other. This interconnectedness, where one author may take an idea, a phrase, a passage, a character or any aspect whatsoever of another author’s work and insert in his own is a literary reference. This reference is recognizably and obviously an influence, device or homage. Literary references can also be made in any form of communication, like oratory song or movies.
In many works, an author may take a character or characters from the vast repository of literary history and use them in his own work. He may base a character from his own work entirely on a character from a previous work. The literary reference may simply be a passing reference to a character from other works or a character could share similar yet recognizable features.
In many types of literary references, an entire plot or structure of a previous work is used in another work. For example, the movie "Apocalypse Now" was based on the plot and structure of Joseph Conrad’s "Heart of Darkness." Many movies and novels have used plots and structures from Shakespeare’s plays to illuminate their own tales, and many plays, novels and films borrow from Greek epics; for example the movie "O Brother, Where Art Thou?" is based on Homer’s "Odyssey."
Literary references can also come in form of straight verse or quotes such as a character quoting a line or verse from a Shakespeare play, in the same way an orator may drop a famous literary line or saying into their speech to make a point or enhance their argument. Some may even be used without knowledge of its literary reference, such as when someone uses the phrase "Catch-22," the title of a Joseph Heller novel.
Sometimes whole scenes or passages can be inserted into works as literary references. A good example of this is the U.S. TV show "The Simpsons," which uses many literary references throughout its work; for example an episode where the main character was depicted on a heroic ancient adventure in Greece was based on many scenes from Greek myths and literature such as "Jason and the Argonauts" and other legends such as Medusa.